relapsing fever

Relapsing fever is a vector-borne disease caused by infection with certain bacteria in the genus '' Borrelia'', which is transmitted through the bites of lice or soft-bodied ticks (genus ''Ornithodoros'').

Signs and symptoms

Most people who are infected develop sickness between 5 and 15 days after they are bitten. The symptoms may include a sudden fever, chills, headaches, muscle or joint aches, and nausea. A rash may also occur. These symptoms usually continue for 2 to 9 days, then disappear. This cycle may continue for several weeks if the person is not treated.

Causes

Louse-borne relapsing fever

Along with '' Rickettsia prowazekii'' and ''Bartonella quintana'', '' Borrelia recurrentis'' is one of three pathogens of which the body louse (''Pediculus humanus humanus'') is a vector. Louse-borne relapsing fever is more severe than the tick-borne variety.

Louse-borne relapsing fever occurs in epidemics amid poor living conditions, famine and war in the developing world. It is currently prevalent in Ethiopia and Sudan.

Mortality rate is 1% with treatment and 30–70% without treatment. Poor prognostic signs include severe jaundice, severe change in mental status, severe bleeding and a prolonged QT interval on ECG.

Lice that feed on infected humans acquire the ''Borrelia'' organisms that then multiply in the gut of the louse. When an infected louse feeds on an uninfected human, the organism gains access when the victim crushes the louse or scratches the area where the louse is feeding. ''B. recurrentis'' infects the person via mucous membranes and then invades the bloodstream. No non-human, animal reservoir exists.

Tick-borne relapsing fever

Tick-borne relapsing fever is found primarily in Africa, Spain, Saudi Arabia, Asia, and certain areas of Canada and the western United States. Other relapsing infections are acquired from other ''Borrelia'' species, which can be spread from rodents, and serve as a reservoir for the infection, by a tick vector.
* '' Borrelia crocidurae'' – occurs in Egypt, Mali, Senegal, Tunisia; vectors – '' Carios erraticus'', '' Ornithodoros sonrai''; animal host – shrew ('' Crocidura stampflii'')
* '' Borrelia duttoni'', transmitted by the soft-bodied African tick '' Ornithodoros moubata'', is responsible for the relapsing fever found in central, eastern, and southern Africa.
* ''Borrelia hermsii''
* '' Borrelia hispanica''
* '' Borrelia miyamotoi''
* ''Borrelia parkeri''
* '' Borrelia turicatae''
* ''Borrelia persica''

''B. hermsii'' and ''B. recurrentis'' cause very similar diseases. However, one or two relapses are common with the disease associated with ''B. hermsii'', which is also the most common cause of relapsing disease in the United States. (Three or four relapses are common with the disease caused by ''B. recurrentis'', which has longer febrile and afebrile intervals and a longer incubation period than ''B. hermsii''.)

Diagnosis

The diagnosis of relapsing fever can be made on blood smear as evidenced by the presence of spirochetes. Other spirochete illnesses (Lyme disease, syphilis, leptospirosis) do not show spirochetes on blood smear. Although considered the gold standard, this method lacks sensitivity and has been replaced by PCR in many settings.

Treatment

Relapsing fever is easily treated with a one- to two-week-course of antibiotics, and most people improve within 24 hours. Complications and death due to relapsing fever are rare.

Tetracycline-class antibiotics are most effective. These can, however, induce a Jarisch–Herxheimer reaction in over half those treated, producing anxiety, diaphoresis, fever, tachycardia and tachypnea with an initial pressor response followed rapidly by hypotension. Recent studies have shown tumor necrosis factor-alpha may be partly responsible for this reaction.

Research

Currently, no vaccine against relapsing fever is available, but research continues. Developing a vaccine is very difficult because the spirochetes avoid the immune response of the infected person (or animal) through antigenic variation. Essentially, the pathogen stays one step ahead of antibodies by changing its surface proteins. These surface proteins, lipoproteins called variable major proteins, have only 30–70% of their amino acid sequences in common, which is sufficient to create a new antigenic "identity" for the organism. Antibodies in the blood that are binding to and clearing spirochetes expressing the old proteins do not recognize spirochetes expressing the new ones. Antigenic variation is common among pathogenic organisms. These include the agents of malaria, gonorrhea, and sleeping sickness. Important questions about antigenic variation are also relevant for such research areas as developing a vaccine against HIV and predicting the next influenza pandemic.

History

Relapsing fever has been described since the days of the ancient Greeks. After an outbreak in Edinburgh in the 1840s, relapsing fever was given its name, but the etiology of the disease was not better understood for a decade. Physician David Livingstone is credited with the first account in 1857 of a malady associated with the bite of soft ticks in Angola and Mozambique.Livingstone D (1857) Missionary travels and researches in South Africa. London: John Murray In 1873, Otto Obermeier first described the disease-causing ability and mechanisms of spirochetes, but was unable to reproduce the disease in inoculated test subjects and thereby unable to fulfill Koch's postulates